text: Biscyclopentadienyl complexes are called metallocenes. A famous example of this type of complex is ferrocene (FeCp), which has many analogues for other metals, such as chromocene (CrCp), cobaltocene (CoCp), and nickelocene (NiCp). When the Cp rings are mutually parallel the compound is known as a sandwich complex. This area of organometallic chemistry was first developed in the 1950s.  Bent metallocenes are represented by compounds of the type [MCpL].  Some are catalysts for ethylene polymerization. Metallocenes are often thermally stable, and find use as catalysts in various types of reactions.
  Mixed-ligand Cp complexes containing Cp ligand and one or more other ligands.  They are more numerous. One widely studied example is the Fp dimer, (CpFe(CO)). Monometallic compounds featuring only one Cp ring are often known as half sandwich compounds or as piano stool compounds, one example being methylcyclopentadienylmanganese tricarbonyl (CpMn(CO)).